In the commercial process for producing polyimides, such as those polyimides sold under the trademark "Kapton" by the Dupont Co., a dianhydride is reacted with a diamine to form an intermediate polyamic acid. The polyamic acid is then spread on a substrate and cured to form the polyimide. For example, the reaction of pyromellitic dianhydride with 4,4'-diamino diphenyl ether to produce a polyamic acid is: ##SPC1##
The polyamic acid cures to form the polyimide and two moles of water. ##SPC2##
The moles of water of condensation given off during the cure creates processing difficulties since it can produce bubbles or blisters in the film as it evaporates.
Also, when the polyamic-acid is stored, some water is formed by closure of the imide ring as during cure. This water then attacks the polymer at another position along the chain which degrades the polymer and reduces its molecular weight. For this reason it is recommended tht the polyamic-acid be refrigerated during storage.